Process for detecting defects in photomasks

ABSTRACT

The present invention provides a process for performing automatic inspection of advanced design photomasks. In a preferred embodiment, an aerial image of a portion of a photomask is generated. A simulated image corresponding to original pattern data is also generated. The aerial image and simulated image are then compared and discrepancies are detected as possible defects.

BACKGROUND OF THE INVENTION

[0001] This invention was made with government support under ContractNo. MDA 972-92-C-0054 awarded by Advanced Research Projects Agency(ARPA). The government has certain rights in this invention.

[0002] The present invention relates to processes for inspectingphotomasks to detect defects. More particularly, the present inventionrelates to an automatic inspection system for detecting defects inphotomasks.

[0003] Advances in capacity in semiconductor chips have generally beenthe result of decreases in the size of features on a chip. The lateraldimensions of features are generally defined by photolithographictechniques in which a detailed pattern is transferred to a photoresistby shining light through a photomask or reticle.

[0004] In recent years, phase shifting masks have been developed toimprove photolithographic processes. Phase shifting masks increasedimage contrast and resolution without reducing wave length or increasingnumerical aperture. These masks also improve depth of focus and processlatitude for a given feature size.

[0005] With phase shift photolithography, the interference of light raysis used to overcome the problems of defraction and improve theresolution and depth of optical images projected onto a target. Withthis technology, the phases of the exposure light at the target iscontrolled such that adjacent bright areas are preferably formed 180°out of phase with each other. Dark regions are thus produced between thebright areas by destructive interference even when defraction wouldotherwise cause these areas to be lit. This technique improves totalresolution at the target.

[0006] Another method that has been developed to produce masks for usein the fabrication of semiconductors containing small features isoptical proximity effect correction (“OPC”). In this method, changes aremade to the binary mask's layout so that it will print more clearly.Because of the limited resolution of the current photolithographic tools(i.e., steppers), the patterns defined on the photomask are transferredinto the resist on the wafer with some distortions referred to asoptical proximity effects. The main consequences in term of line widthcontrol are: corner rounding, difference between isolated andsemi-isolated or dense patterns, lack of CD linearity or where smallfeatures print even smaller than their expected size compared to largefeatures, and line end shortening where the length of a line having asmall line width becomes smaller than its expected size.

[0007] Moreover, optical proximity effects are convoluted withsubsequent processing step distortions like resist processing, dry etchproximity effects and wet etch proximity effects. In order to achieve asufficient line width control at the wafer level, the mask designs arecorrected for proximity effects, namely re-entrant and outside serifsare used to correct rounding and the edges of the patterns are moved tocorrect line width errors. Another technique consists in adding small,non-printing features, referred to as subresolution features, in orderto correct line width errors. In some cases, these features can alsoimprove the process latitude of the printed resist patterns.

[0008] Printable defects in photomasks and reticles have historicallybeen a source of defects that have reduced die yields. With currentphotolithographic techniques, printable defects in the photomasks, arerepeated many times over the surface of a semiconductor wafer and canresult in substantial yield losses. Accordingly, it is important todetect and correct as many defects as possible in the photomasks.

[0009] Defects in photomasks can arise from many different sources. Forexample, certain defects such as bubbles, scratches, pits and fracturescan be contained in the raw glass substrates. Defects can also be formedin the chrome layer by particulate inclusions, pin holes or voids, andexcess material.

[0010] As advances have been made in photomask design such as phaseshifting and OPC, it has become harder to detect defects in thephotomasks. However, defect detection and correction has becomeincreasingly important. Previously, masks were checked by exposing anddeveloping an image on a resist layer on a plain quartz wafer. Theresulting pattern was then inspected. However, there was nodie-to-database inspection with this system.

[0011] Automatic photomask defect detection systems have been developedand are commercially available. These include systems such as the KLARISsystem by KLA Instruments Corp. and the Chipcheck system by CambridgeInstruments. Inspection tools such as KLA and Orbot systems are alsoavailable for die-to-die inspection of the printed image on wafers.These systems are limited by the fact that the inspection is performedat 1× (versus 4× or 5× for most advanced reticles). The maximumallowable defect size is smaller and a complete inspection is notpossible in the case of a single die reticle as die-to-databasecapability is not available on these systems.

[0012] In the KLA system, light is transmitted through the photomask anddetected by a CCPD image sensor. This image is then compared to theimage from a database or compared to the image from another die on themask. If one comparison of a die to the database is performed, theremaining comparisons on the mask can all be die-to-die inspections thatrelate back to the initial comparison.

[0013] These prior art systems are generally limited to basic maskdesigns and have limited capability of checking advanced designs such asthose containing optical proximity effect corrections and phase shiftinglayers.

[0014] Because of the importance in detecting and correcting photomaskdefects, it would be a significant advancement in the art to provide anautomatic process for detecting defects in advanced photomask designs.Such a process is disclosed and claimed herein.

SUMMARY OF THE INVENTION

[0015] The present invention provides an automatic process for detectingprintable defects in masks. The invention is particularly useful inanalyzing advanced photomask designs such as those which include opticalproximity effect corrections and phase shifting layers.

[0016] In a preferred embodiment, a mask design is generated from abinary mask layout. The mask design is then used to generate a photomasksuch as by suitably etching a chrome layer on a quartz plate. Thepresent invention provides a process for detecting any defects that areformed in the photomask. In a preferred embodiment, an aerial image ofthe photomask is generated. This is then compared with a simulated imageof the binary mask layout which has been adjusted to account forexpected distortions and corner rounding caused by image processing ofthe mask and wafer. Any discrepancies between the aerial image and thesimulated image are likely due to defects in the photomask.

[0017] In a second preferred embodiment, the aerial image of thephotomask is compared with a simulated aerial image of the mask design.Again, any discrepancies between these two images are likely due todefects in the photomask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic illustration of a feature of a photomaskdesign illustrated at different stages according to a first embodimentof the present invention.

[0019]FIG. 2 is a schematic illustration of a feature of a photomaskdesign illustrated at different stages according to a second preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention provides a process for performing anautomatic inspection of advanced design photomasks to detect printabledefects which might cause fatal flaws in semiconductor dies. Theinvention is best understood by reference to the attached drawings inwhich like parts are designated with like numerals.

[0021] Referring first to FIG. 1, a feature of a semiconductor maskdesign is generally designated at 10. Feature 10 forms part of a binarymask layout. From this layout, features on an advanced mask design aregenerated. Feature 12 corresponds to feature 10 but is obtained byapplying optical proximity effect correction techniques to feature 10.Feature 12 is then used to generate a corresponding feature on aphotomask.

[0022] Feature 14 corresponds to feature 12 as it appears in the chromeon the photomask. During fabrication, a defect 16 was formed in thedesign. Defect 16 comprises excess chrome which remains on the quartzplate. However, it will be appreciated by those skilled in the art thatthe process of the present invention can also be used to detect othertypes of defects such as missing chrome, contamination, glass damage,phase defects, transmission errors and even poor repairs made to adefective mask.

[0023] In order to detect any defects, an aerial image 18 is generatedfrom feature 14 on the photomask. Aerial images can be generated using asystem comparable to the commercially available MSM-100 aerial imagemeasurement system manufactured by Carl Zeiss, Inc. This system is setup to analyze actual masks under optical conditions that are essentiallyequivalent to those of a stepper of interest, but greatly magnified. Asthe exposure light is shown through the mask and magnified, a UVsensitive CCD camera is used for data capture.

[0024] A simulated image 20 of feature 10 is also generated and takesinto account expected distortions and corner rounding due to imageprocessing of the mask and wafer.

[0025] Image 20 can also be the result of the convolution of feature 10with some convolution function(s) representing, but not limited to, theaerial image, the mask fabrication process and OPC corrections. Forexample, the aerial image can be generated by various software programssuch as FAIM produced by Vector Technologies, DEPICT produced by TMA,and SPLAT produced by The University of California, Berkeley.

[0026] Aerial image 18, which is generated using a threshold such thatdimensions of image 18 match the dimensions of image 20, is thencompared to simulated image 20. Incongruity 24, which corresponds todefect 16, will be identified during the comparison as a discrepancybetween the two images.

[0027] Reference is next made to FIG. 2 which illustrates a secondpreferred embodiment of the present invention. In this embodiment, afeature 10 of binary mask design is again used to generate a mask designfeature 12. This mask design is then used to generate feature 14 on aphotomask and an aerial image 18 is generated from the image on thephotomask.

[0028] However, in this embodiment, simulated image 30 is generated as asimulated aerial image of mask design image 12. Aerial image 18 is thencompared to this simulated image 30 to obtain a comparison 32 where anyincongruities 34 will appear as discrepancies between the two images.Image 30 can also be the result of the convolution of feature 10 withsome convolution function(s) representing, but not limited to, theaerial image, the mask fabrication process, OPC corrections, etc.

[0029] While the invention has been described with respect to maskdesigns using optical proximity effect correction techniques, it will beappreciated by those skilled in the art that it can also be applied toother advanced mask designs such as those using phase shifting layers.The invention can be used either to analyze known defects or to do anautomated inspection over an entire mask surface. Additionally, whilethe above description has been limited to the analysis of a singlefeature, it will be appreciated that blocks of multiple features can beanalyzed.

[0030] In addition to photomasks, the present invention can be used forx-ray masks, stencil masks for ion projection lithography, masks forelectron beam projection lithography, etc. The techniques of the presentinvention can also be applied to imaging systems other than those usedin the manufacture of integrated circuits.

[0031] While the invention has been described with respect to thepresently preferred embodiments, it will be appreciated by those skilledin the art that changes and modifications could be made to the disclosedembodiments without departing from the spirit or scope of the invention.For example, the inspection technique and aerial images could beperformed out of focus in order to detect defects that mainly print outof focus such as phase defects. Additionally, other techniques ofmathematical processing of the data can be used to generate images 20and 30. Further, the simulated image of the mask can be generated duringinspection or a portion of the simulation can be performed beforeinspection and the remainder during inspection. Accordingly, all changesor modifications which come within the meaning and range of equivalencyof the claims are to be embraced within their scope.

We claim:
 1. A process for detecting defects in masks comprising:generating an aerial image of a portion of a mask; generating asimulated image corresponding to original pattern data used to createsaid mask; and comparing said aerial image to said simulated image.
 2. Aprocess for detecting defects in masks as defined in claim 1 whereinsaid simulated image is generated from original pattern data taking intoaccount expected distortions and corner rounding due to imageprocessing.
 3. A process for detecting defects in masks as defined inclaim 1 wherein said simulated image is obtained by generating an aerialimage of a mask design used to generate a portion of the mask with whichit is compared.
 4. A process for detecting defects in masks as definedin claim 1 wherein said mask is generated using proximity effectcorrection techniques.
 5. A process for detecting defects in masks asdefined in claim 4 wherein said mask is generated using opticalproximity effect correction techniques.
 6. A process for detectingdefects in masks as defined in claim 4 wherein said mask is generatedusing x-ray proximity effect correction techniques.
 7. A process fordetecting defects in masks as defined in claim 4 wherein said mask isgenerated using ion beam proximity effect correction techniques.
 8. Aprocess for detecting defects in masks as defined in claim 4 whereinsaid mask is generated using e-beam proximity effect correctiontechniques.
 9. A process for detecting defects in masks as defined inclaim 1 wherein said photomask includes phase shifting techniques.
 10. Aprocess for detecting defects in masks as defined in claim 1 whereinsaid mask includes proximity effect correction techniques and phaseshifting techniques.
 11. A process for detecting defects in masks asdefined in claim 1 wherein said mask comprises a photomask.
 12. Aprocess for detecting defects in masks as defined in claim 1 whereinsaid masks are used in the manufacture of integrated circuits.
 13. Aprocess for detecting defects in masks as defined in claim 1 whereinsaid mask comprises an x-ray mask.
 14. A process for detecting defectsin masks as defined in claim 1 wherein said mask comprises a stencilmask for ion projection lithography.
 15. A process for detecting defectsin masks as defined in claim 1 wherein said mask comprises a mask forelectron beam projection lithography.
 16. A process for detectingdefects in masks as defined in claim 1 wherein said aerial image andsaid simulated image are generated out of focus.
 17. A process fordetecting defects in photomasks comprising: generating an aerial imageof a portion of a photomask; generating a simulated image correspondingto original pattern data used to create said photomask; and comparingsaid aerial image to said simulated image.
 18. A process for detectingdefects in photomasks as defined in claim 16 wherein said simulatedimage is generated from original pattern data taking into accountexpected distortions and corner rounding due to image processing.
 19. Aprocess for detecting defects in photomasks as defined in claim 16wherein said simulated image is obtained by generating an aerial imageof a mask design used to generate the portion of the photomask withwhich it is compared.
 20. A process for detecting defects in photomasksas defined in claim 16 wherein said photomask is generated using opticalproximity effect correction techniques.
 21. A process for detectingdefects in photomasks as defined in claim 16 wherein said photomaskincludes phase shifting techniques.
 22. A process for detecting defectsin photomasks as defined in claim 16 wherein said photomask includesproximity effect correction techniques and phase shifting techniques.23. A process for detecting defects in photomasks as defined in claim 16wherein said aerial image and said simulated image are generated out offocus.
 24. An apparatus for detecting defects in photomasks comprising:an aerial image measurement system for generating an aerial image of aportion of a photomask; a simulated image generating system forgenerating a simulated image corresponding to original pattern data ofsaid photomask; and a comparator for comparing said aerial image andsaid simulated image.
 25. An apparatus for detecting defects inphotomasks as defined in claim 22 wherein said image simulator comprisesan aerial image measurement system.
 26. An apparatus for detectingdefects in masks comprising: means for generating an aerial image of aportion of a mask; means for generating a simulated image correspondingto original pattern data used to create said mask; and means forcomparing said aerial image with said simulated image.
 27. An apparatusfor detecting defects in photomasks comprising: means for generating anaerial image of a portion of a photomask; means for generating asimulated image corresponding to original pattern data used to createsaid photomask; and means for comparing said aerial image with saidsimulated image.